This invention relates to improvement in a method of fabricating uranium oxide powder suitable for the production of nuclear fuel starting from uranium hexafluoride more particularly to improvement in an ammonium diuranate method.
The ammonium diuranate (ADU) method is widely known as a method for producing from uranium hexafluoride (UF.sub.6) uranium dioxide (UO.sub.2) powder which is suited for the production of nuclear fuel. Most popular example of the ADU method is a method which comprises reacting UF.sub.6 gas with water to form an aqueous uranyl fluoride (UO.sub.2 F.sub.2) solution, reacting the aqueous UO.sub.2 F.sub.2 solution with ammonia (NH.sub.3) to precipitate ADU, filtering, drying, calcining and reducing the ADU precipitate to convert the thus processed ADU precipitate to UO.sub.2 powder.
In the above-mentioned conventional method ADU is produced by reaction steps as shown in the following reaction schemes. ##STR1##
In the above method, as shown in the reaction scheme (I), there is 4 moles of hydrogen fluoride (HF) per mole of uranium (U) in the aqueous UO.sub.2 F.sub.2 solution prepared by the hydrolysis of UF.sub.6. In this case, in the production of ADU as shown in the reaction scheme (II), at first ammonium fluoride (NH.sub.4 F) is formed by neutralization with HF and as the result the ADU thus formed is in the form of primary grains whose grain size is relatively large. The greater the grain size of the primary grains of ADU the greater the grain size of the primary grains of UO.sub.2 powder prepared from the ADU, which means that the UO.sub.2 powder has a relatively low activity. When sintered pellets for nuclear fuel are produced from such UO.sub.2 powder with a relatively low activity the sintered density of the pellets which can be produced under generally employed conditions is about 95% TD (theoretical density) and the grain size of the sintered pellet is about 10 micrometers.
Examples of the conventional ADU methods include, in addition to the above-described method in which ADU is produced from an aqueous UO.sub.2 F.sub.2, a method in which ADU is produced from an aqueous uranyl nitrate (UO.sub.2 (NO.sub.3).sub.2 solution. This method comprises hydrolyzing UF.sub.6 gas in a nitric acid solution of a defluorinating agent to form an aqueous UO.sub.2 (NO.sub.3).sub.2 solution, purifying the solution by solvent extraction, reacting the purified solution with ammonia to produce ADU, and then converting ADU to UO.sub.2 powder in the same manner as in the above-described ADU method. With this method it is possible to obtain UO.sub.2 powder having a very high activity and in this case it is possible to obtain sintered pellets having a high sintered density as high as 99% TD. However, with this highly active UO.sub.2 powder the grain size of the sintered pellets to be obtained is at most in the order of 20 micrometers. In order to obtain sintered pellets with greater grain size from UO.sub.2 powder obtained by this type of ADU method it is necessary to take various measures such as use of elevated sintering temperature, prolonged sintering time, etc., which are however practically difficult to achieve.
When using UO.sub.2 pellets as nuclear fuel it is necessary for the fuel to burn stably during irradiation. In this case, one of the indices for showing the state expressed by the term "burn stably" is the fission product gas (FP gas) is retained within the pellet as much as possible, or as little as possible an FP gas is released from the pellet. From the results of irradiation tests a tendency has been confirmed that the greater the grain size of the pellet the more excellent the ability of the pellet to retain FP gas. However, when the pellet has too great a grain size there is a possibility that there occurs harmful effects such as reduction in the mechanical strength, and therefore the grain size must be selected or adjusted appropriately. Although it has not yet been clarified completely as to which range of crystal grain size is appropriate it is presumed practically sufficient to use pellets with a grain size of up to 100 micrometers which is set up as a standard for the present. When UO.sub.2 powder obtained by the conventional ADU method is used it is difficult to produce UO.sub.2 pellets with a crystal grain size of greater than 20 micrometers in ordinary sintering methods and it is further difficult to produce UO.sub.2 pellets having an appropriate crystal grain size.